Chemistry

Atoms and Molecules


Molecular Masses of Some Common Elements


ElementSymbolAtomic
mass		Molecular
formula		Molecular mass
1.Hydrogen
2.Nitrogen
3.Oxygen
4.Chlorine		H
N
O
Cl		1 u
14 u
16 u
35.5 u		H2
N2
O2
Cl2		2x1 = 2 u
2 x 14 = 28 u
2 x 16 = 32 u
2 x 35.5 = 71 u

We will now solve a problem based on the calculation of molecular mass of a compound.
Sample Problem. Calculate the molecular mass of sulphuric acid (H2SO4).
(Given : Atomic masses : H = 1 u ; S = 32 u ; O = 16 u).
Solution. The molecular formula of sulphuric acid is H2SO4.This formula shows that one molecule of sulphuric acid (H2SO4) contains 2 atoms of hydrogen,1 atom of sulphur and 4 atoms of oxygen. So, the molecular mass of H2S04 will be equal to the sum of the masses of 2 hydrogen atoms, 1 sulphur atom and 4 oxygen atoms. Knowing that the atomic mass of hydrogen is 1 u, of sulphur is 32 u and that of oxygen is 16 u, the molecular mass of sulphuric acid can be calculated as follows:
Molecular mass of H2SO4 = Mass of 2H
atoms + Mass of S
atom + Mass of 4 'O'
atoms
= 2x1+32 + 4x16
= 2 + 32 + 64
= 98 u
Thus, the molecular mass of sulphuric acid is 98 u.
Before we conclude the discussion on formulae, it will be good to note the significance of the chemical formula of a substance which is given below.

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Notes
Atoms and Molecules - Notes
1. Drawbacks of Dalton’s Atomic Theory
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2. Monovalent Cations (Cations Having a Valency of 1+)
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3. Divalent Cations (Cations Having a Valency of 2+)
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4. Dalton’s Symbols of Elements
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5. Trivalent Cations (Cations Having a Valency of 3+)
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6. Formulae of Some Molecular Compounds
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7. an example, let us give the significance of symbol C
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8. An Important Discussion
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9. Chemical Formulae
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10. Molecular Formulae of Some Common Elements
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11. As an example, let us give the significance of the formula H2O
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12. 2. A Negatively Charged Ion is Known as Anion
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13. Molecules
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14. Explanation of the Law of Conservation of Mass
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15. Molecules of Compounds
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16. Molecular Mass
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17. Atoms
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18. Some Ionic Compounds
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19. Atoms, Molecules and Ions
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20. Formulae of Elements
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21. Simple Ions and Compound Ions (Polyatomic Ions)
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22. Formulae of Compounds
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23. Formula Mass
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24. Symbols of Elements
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25. 1. A Positively Charged Ion is Known as Cation
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26. 1. Molecules of Elements
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27. Mole of Molecules
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28. Gram Atomic Mass And Gram Molecular Mass
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29. Mole of Atoms
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30. John Dalton
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31. Ions
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32. Dalton's Atomic Theory
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33. Explanation of the Law of Constant Proportions
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34. Significance of the Formula of a Substance
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35. Some Common Ions
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36. Molecular Masses of Some Common Elements
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37. Laws Of Chemical Combination
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38. Divalent Anions (Anions Having a Valency of 2-)
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39. Calculation of Molecular Mass
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40. Ionic Compounds
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41. Formula Unit of Ionic Compounds
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42. Atoms And Molecules
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43. Writing Of Formulae Of Ionic Compounds
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44. How Do Atoms Exist
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45. Gram Molecular Mass
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46. Problems Based On Moles Of Molecules
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47. Laws Of Conservation of Mass
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48. Experiment to Verify Law of Conservation of Mass
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49. Problems Based On Moles Of Atoms
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50. Writing Of Formulae Of Molecular Compounds
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